1. Field of the invention
This invention relates in general to a tieback apparatus for a subsea well, and in particular to a means for sealing the tieback connector and testing the seal.
2. Description of the Prior Art
When drilling a subsea well, mudline tieback connections are employed, particularly for exploratory wells. In a mudline tieback connection, rather than utilize a subsea pressure containing wellhead, all of the casing strings will extend to the drilling rig. The pressure control will thus be at the drilling rig rather than subsea. Tieback connectors will be located at the subsea wellhead housing to connect the casing strings that are cemented in place to tieback strings that extend to the surface drilling vessel.
If a well is be produced, the operator may temporarily abandon the well after drilling by removing the tieback strings extending from the casing strings to the surface. The operator will place a cap on the subsea structure until he later returns for running production equipment.
The tieback connector for each string includes an upward facing tubular tieback member located at the subsea wellhead at the upper end of each casing string that will serve subsequently to be connected to a tieback string for production. This lower tieback member has a set of running grooves, normally left-hand threads, that were utilized when the casing string was initially run. Also, the lower tieback member will have a set of tieback grooves, which may be threads or grooves, for securing to a subsequent tieback string when it is desired to complete the well for production.
When it is desired to complete the wells for production, the operator will position a production platform over the well or wells. The operator removes the cap from the lower tieback member and lowers a tieback string back into engagement with the lower tieback member. The tieback string has an upper tubular tieback member on its lower end that stabs into and latches with the lower tieback member. The upper and lower tieback members have a seal that seals the connection from the annulus and bore. The upper and lower tieback members make up the tieback connector.
Then, the operator must test the tieback connection to assure that the tieback connector does not leak to the annulus. In the past, an operator would lower an isolation test tool which had an upper elastomeric seal and a lower elastomeric seal. He would set the upper seal in the upper tieback member bore and the lower seal in the casing below the lower tieback member. The operator would apply pressure to this area to determine if leakage exists.
One problem with this test is that the lower seal of the isolation tool must engage an internal bore within the casing in an area that has been previously contacted by a drill pipe. This surface may be damaged and difficult to seal against. Consequently, it is difficult to obtain an accurate test.